1. Field of the Invention
This invention relates to objective lens systems having multiple lenses of crystalline semiconductor materials such as silicon and germanium. The invention lens is particularly adapted for use in light weight portable FLIR (Forward Looking Infrared) surveillance systems. 2. Description of the Prior Art
Single field-of-view, four-element infrared objective lens systems find particular application in scanning infrared systems. These systems typically employ a detector upon which an image is focussed by the invention lens system. The detector used is typically flat. Prior art lens designs are best suited for projecting an image on a curved field of curvature. The present selectable field-of-view infrared objective lens is designed to provide a narrow or a wide field-of-view, to minimize narcissus on the detector, and form a flat field of curvature.
Infrared objective lens systems, used to project an image through a cold stop onto a flat detector to form an image plane in a cryogenically cooled space, have the problem of re-imaging the detector onto itself. The mirror image of itself is due to reflections from the surfaces of lenses within the objective lens system. The image of the detector when sensed by the detector along with the intended image results in a loss of resolution. Re-imaging of the detector onto the detector by the infrared lens is an effect referred to as narcissus. The invention selectable field-of-view infrared objective lens is designed to provide alternative fields-of-view and effectively minimize the effect of narcissus while maintaining a flat field of curvature.
An application for U.S. Patent entitled "Selectable Field-of-View Infrared Lens", Ser. No. 293,763, was filed on Aug. 17, 1981 by the same inventor. This application discloses a reflective afocal telescope coupled to an objective lens system. The reflective afocal telescope uses mirrors not included in the present invention.